1. Field of Invention
The present invention pertains to downhole nuclear magnetic resonance (NMR) tools for use in a wellbore, particularly small-scale downhole NMR tools.
2. Related Art
Downhole NMR tools are commonly used, in oil and gas exploration, for example, to ascertain or infer properties of the subsurface formations encountered by a wellbore. Downhole NMR tools may be used while drilling the wellbore, or may be run into the wellbore after drilling, for example, on a wireline. Various reservoir fluid properties can be measured using a downhole NMR tool. Measurements can be made on reservoir fluids in the formation, or measurements can be made on a fluid sample withdrawn from the formation or wellbore. The NMR tool can be a stand-alone tool or may be incorporated as a module in a fluid sampling tool such as that disclosed in U.S. Pat. No. 6,346,813 B1 issued to Kleinberg. An example of a formation fluid tester tool is the Modular Formation Dynamics Testing tool marketed under the trade name of MDT™ by Schlumberger Technology Corp. (Houston, Tex.).
A downhole NMR tool generally includes a magnet that produces a static magnetic field over the volume of the fluid sample. The NMR tool also includes a coil or antenna to produce radio frequency (RF) pulses. The magnetic dipole moment of the RF antenna is substantially perpendicular to the magnetic dipole moment of the static magnetic field. In addition, the NMR tool may include one or more gradient coils.
The static field of a downhole NMR tool is generally too inhomogeneous to allow NMR spectroscopy to be performed. The inhomogenieties are attributed to variations in the magnetic material comprising the magnets and the magnet configuration. Thus, the static magnetic field inhomogenieties over the sample volume are to too large to perform NMR spectroscopy, but is generally acceptable for conventional NMR measurements such as relaxation times and diffusion.